The invention relates to implanted medical devices, and in particular, implanted medical devices that monitor cardiac signals.
In the medical fields of cardiology and electrophysiology, many tools are used to assess the condition and function of a patient""s heart from observed frequency, polarity and amplitudes of the PQRST complex associated with a heart cycle. One such tool is the electrogram (EGM), which is an electrical signal from a region of the heart. An electrogram may be used by many implantable devices that monitor cardiac signals, such as a cardiac pacemaker or an implantable cardioverter/defibrillator (ICD).
An EGM that records the activity of an atrium of the heart is called an atrial EGM, or A-EGM. An A-EGM signal usually includes one prominent peak magnitude corresponding to atrial depolarization, also known as the P-wave. An EGM that records the activity of a ventricle of the heart is called a ventricular EGM, or V-EGM. A V-EGM usually includes one prominent peak magnitude as well, but the peak of the V-EGM corresponds to ventricular depolarization, also known as the R-wave.
Detection of P-waves and R-waves is important in sensing cardiac rates and rhythms. Bradycardia, tachyarrhythmia, premature atrial contraction, premature ventricular contraction, heart block and fibrillation are some of the conditions that may be monitored through sensing P-waves, R-waves, or both.
In a typical cardiac monitor, A-EGM signals and V-EGM signals are filtered and amplified, and are then compared to a sense threshold. When the filtered A-EGM signal exceeds the atrial sense threshold, the implanted device generates a signal indicating that a P-wave has been detected. Similarly, when the filtered V-EGM signal exceeds the ventricular sense threshold, the implanted device generates a signal indicating that an R-wave has been detected.
In some patients, the magnitudes of the cardiac signals change over the long term. The peaks of a V-EGM signal in a patient, for example, may decrease over a period of months. The downward change of signal magnitudes may have many causes.
The prior art includes many techniques for monitoring cardiac signals. For example, U.S. Pat. No. 4,513,743 to van Arragon et al. describes storing and presenting data such as peak QRS amplitudes and peak P amplitudes, and arranging the data for presentation in a histogram. Similarly, U.S. Pat. No. 5,722,999 to Snell describes a system for acquiring and displaying medical data, such as graphing R-wave amplitudes over several months.
Monitoring signal magnitudes relative to sense thresholds has also been described in various contexts. U.S. Pat. No. 4,708,144 to Hamilton, et al., for example, discloses adjusting pacemaker sensitivity in response to peak R-wave values. U.S. Pat. No. 5,330,513 to Nichols et al. describes a sensing threshold analysis that involves monitoring average peak values over a programmed time interval.
U.S. Pat. No. 5,513,644 to McClure et al. describes short-term monitoring of peak values of electrogram signals, and automatically adjusting system sensitivity in response to the signals. U.S. Pat. No. 5,957,857 to Hartley discloses adjustment of amplifier gain in response to peak values of P-waves or R-waves. U.S. Pat. No. 6,192,275 to Zhu et al. describes adjusting the sensitivity of an evoked response threshold, in response to fluctuations in R-wave amplitudes due to factors such as respiration and activity level.
Signal processing techniques related to processing EGM signals are well known. In addition to the patents described above, U.S. Pat. No. 5,350,411 to Ryan et al. illustrates how an A-EGM signal may undergo different processing operations in parallel. V-EGM signals likewise may be processed along different paths simultaneously.
The invention is directed to techniques for tracking the magnitudes of representative filtered EGM signals so that the magnitudes can be monitored over time. Usually the monitoring is over several days at least. The signal that is monitored is not a raw EGM signal, but rather a filtered EGM signal having the same frequency components as the EGM signal supplied to the sense amplifier.
The invention provides techniques for generating digital peak values. From a group of digital peak values generated in a time period, such as one day, a representative digital peak value is selected.
Peak values can be captured by signal processing circuitry that includes filters, a peak detector, a sample-and-hold circuit that captures the magnitude of the peak, and an analog-to-digital converter that generates a digital peak value as a function of the magnitude of the peak. A processor may validate the peak values and select a peak value as representative of the plurality of peak values. The representative peak value may be selected from the plurality of peak values, for example, or may be selected by mathematical techniques.
In an exemplary implementation, five digital peak values are generated each day. Each digital peak value comes from a single cardiac signal and represents a true intrinsic sensed event. From this collection of five digital peak values, one is selected as the representative peak value of the day. For example, the median value among the five digital peak values may be selected as the representative peak value of the day.
By comparing representative peak values for several time periods, changes in signal strength and efficacy can be identified. Of particular concern is signal attenuation, i.e., a downward change of signal magnitudes. Substantial attenuation of peak values may lead to undersensing of valid P-waves or R-waves, and cause detection failure. In many cases, attenuation may be indicative of changing conditions such as a change in the position of the sensing electrode, a failure of the electrode or a change in the cardiac tissue.
The representative peak values over time may be monitored. If a representative peak value falls below an alert threshold, an alert may be generated notifying the patient and/or the patient""s physician of a potential decline in signal efficacy. In addition, several peak values may be presented in a format that allows the values to be compared to each other over time. The physician may use this data to diagnose the condition of the patient, conduct additional diagnostic tests, adjust the therapy for the patient or perform some other appropriate action. In addition, the implanted device may adjust sensitivity to EGM signals by adjusting sense thresholds. An advantage of the invention, therefore, is that it automatically collects data that assists the patient and the physician in diagnosing physiologic conditions, establishing therapy, and troubleshooting the implanted device.
The above summary of the invention is not intended to describe every embodiment of the invention. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.